Despite efforts spanning three decades, there is no licensed vaccine against respiratory syncytial virus (RSV), the most important cause of infant pulmonary disease worldwide. Bovine respiratory syncytial virus (BRSV) is antigenically related to RSV, causes no significant pathology in laboratory animals, and confers highly significant protection against RSV infection in seronegative animals. It appears not to cause potentiated disease, unlike previous candidate vaccines. Because virtually all potential recipients of an RSV vaccine would be infants possessing substantial amounts of maternally derived antibody to RSV, and because such antibody has been shown to block the efficacy of other candidate vaccines, the specific aim of this proposal is to test the efficacy and safety of BRSV in cotton rats previously administered human RSV antibody in amounts equivalent to those seen in human infants. If BRSV protects against RSV under these conditions, followup studies would include clinical trials.